Method of measuring mechanical forces



,1937. w. JANOVSKY4 2,081,748

METHOD OF MEASURING MECHANICAL FORCES Original Filed June 26, 1933 ffibrn gs.

EEABMJMS METHOD F MEASURING BWCHANIGAL FQRQES Wilhelm Janovslky, Berlin-Spandan, Germany, assignor to Siemens & Halskc Aliticngesell-e schaft, Berlin-Siemensstadt, Germany, a corporation of Germany omen Driginal application June 26, race, Serial No.

677,"?05. Divided and this application Novem- '1 her 21, 1935, Serial No. 50,835. In Germany November 26, 1932 3 Claims. (Cl. Mi -35B.)

The present application is a division of my Fig. 2 shows how the inductivity L varies with pending application filed in the United States increasing exciting current intensity, that is on Patent Ofiice on June 26, 1933, Serial No. 677,705, the one hand in the unstrained condition (1 :0) which has matured into Letters Patent No. and, on the other hand, in the strained condition 5 2,053,560 of September 8, i936. Pm of the test body. The current intensities This invention relates to an improvement in J1, J2, J3 in accordance with Fig. l are also the so-c-alled magneto-elastic method of measurindicated in Fig. 2 which latter indicates a uring mechanical forces. In such method, a body e Current intensity 4, as W s n exc tin of magnetizable material is magnetized by means current intensity J14, corresponding to the coercive id of an exciting coil connected to a source of alterforce of the material concerned, the relation of 1o hating current, said body being also at the same these current intensities being J1 Jk J2 J3 J4. time subjected in its entirety or in certain parts, Now, if, in addition, the difference of the values to the forces to be measured. The strain proof L is also plotted for P==O and P=Pm respec duced by the said forces affects the permeance tively, one obtains the curve designated by AL,

is of the material under test and thereby also which shows amaximum for a value of the excitchanges the impedance of the exciting coil. Iting current intensity between J}; and J2.

follows that the magnitude of the forces to be The difference of t d t ve eact nc measured can be determined by measuring the ARzwhL corresponding to AL and showing, of

impedance of the exciting coil with the aid of course, a maximum at the same point, had been an electrical measuring device. I have found plotted in Fig. 3. But further consideration go that it is advantageous to employ a magnetizashows that the test voltage V available for the tion exceeding the degree used previous to my inmeasurement, in say a compensation device, corventio'n, and particularly to employ, for magresponds to the product JAR, equally plotted. in netizing the body under test, an alternating cur- Fig. 3. It may be recognized from the graph rent or such character that the magnetization that the peak value of the measuring voltage V 25 will exceed the limit given by the coercive force of lies in the neighborhood of the intensity J3. The the material of said body. curve designated'by J AR represents a measure Reference is to be had to the accompanying of the corresponding power whose maximum drawing in which Figs. 1, 2, and 3 are diagrams appears to be considerably more shifted in terms illustrating the electrical and magnetic condiof a stronger magnetization. 3o tions involved in my invention, and Fig. 4 is a dia- The grap Show that an exciting C re t grammatic view showing, as an example, one of tensity exceeding the value 5n oiiers the further the circuit arrangements which may be used in advantage that the voltage available for the carrying out my invention. measurement, viz, J .AR is still further increased.

Fig. i represents the variation in inductivity It may in some cases evenbe advantageous-to 35 L of a test body magnetized by different exciting make the exciting current intensity higher than currents J1, J2, J3 under the action of variable the intensity J3 corresponding to the maximum mechanical forces P. It will be seen that in all of J .AR, so as to reach the value J4, in order to the cases the inductivity decreases as the strain come into the range of maximum power J AR.

so on the test body increases from P=O to P=Pm. This will more particularly be advisable, if small 0 For the current intensity J1 which is in accordforces are to be measured and correspondingly ance with the degree of magnetization used up the cross-sectional area of the test body or its to now, different characteristics are obtained with volume must be chosen relatively small since, in increasing and decreasing strain, on account of this case, the available power is also small.

hysteresis phenomena. In practice these phe- Another important advantage lies in the in- 45 nomena disappear, however, entirely if, according creased magnetization insofar as, with a suitably to the invention, the intensity of the exciting chosen exciting current intensity, the variations current is increased beyond the limit determined in the exciting voltage may, to a certain extent, by the coercive force of the material. As may be compensated for. For this end it is preferable be seen from the curves designated by J2 or J3 as to make use of an intensity in the neighborhood 50 corresponding to higher intensities exceeding the of J3, as the curve of the test voltage JAR shows limit of the coercive force, measurement'results in this case a maximum and, due to variations are obtained independently of whether the same in voltage, intensity changes have the least inmeasurements are effected with increasing or defluence on the height of the voltage J .AR correcreasing forces P. spending to the force under test. 55

As has already been mentioned, the permeance f the test body is preferably measured by means of a compensation method, e. g. in the shape of a Wheatstone bridge. The magnitude of the impedance of the exciting winding of the test body is in this case compared with that of a known resistance. It is of special advantage touse a Wheatstone bridge balanced in the unstrained condition of the test body, so as to enable the current flowing through the measuring instrument in the diagonal branch to disappear, the deflection on the measuring instrument being thus a measure of the forces or momenta straining the test body under test.

Fig. 4 schematically shows a D. C.-compensation device. The A. C. source I is connected through a series resistance I 6 to the branching points of the bridge, in the arms of which there are. connected two rectifiers I1 and I8 respectively, the exciting Winding 2 0f the test body I and a suitable adjustable reference resistance l9. At 3 I have indicated a yoke or shell in contact with'the test body or core I. This shell completes the magnetic circuit. A D. C. measuring instrument lies in the diagonal branch of the bridge. The test body or core I is preferably made of nickel-iron and carries the exciting winding or coil 2. This body is in contact with a shell 3 which completes the magnetic circuit, said shell consisting for instance of an iron band or wire wrapping or being built up of individual sheets. At 4 I have indicated a suitable support or abutment to engage one end of the test body I.

The measurement is eifected by first adjusting the reference resistance III in such a manner as to cause the instrument 20 to give no deflection in the unstrained condition of the test body. In this case, the amount oi the resistance I8 is equal to that of the impedance R of the exciting winding 2 of the test body. If now the test body I is strained by the force under test P, for instance through the medium of a pressure member 5, the magnitude of the impedance R of the exciting winding 2 varies and the. instrument 20 gives a deflection corresponding to this variation AR which may be taken as a measure of the force P under test. Since, for the measurement, the simple magnitudes of the resistances are taken into account, regardless of the phase condition of the voltages, use can be made of a simple ohmic resistance, as a reference resistance, and a special phase compensation can be dispensed with. It.must, however, be admitted that, in this case, the instrument lying in the diagonal branch of the bridge is also traversed by an A. C. component. If this is considered to be objectionable, means well known in the art may be used also to balance the phases of the impedances of the exciting winding of the test body and of the reference resistance which are to be compared, so asto enable boththe direct current and the alternating current to disappear in the diagonal branch of the measuring device.

be opposed to sign. The product JAR Due to the variation in impedance AR of the exciting winding, the exciting current intensity J in the winding would also vary with constant voltage of the A. C. source, but these variations would responsible for the magnitude of the instrument deflection would thereby be reduced. To avoid this, the bridge should be connected across resistance I 6 which should be chosen sufliciently high to enable the variable impedance of the exciting winding to become negligible as compared therewith. The exciting current intensity remains therefore approximately constant irrespective of the strain on the test body.

While Fig. 4 illustrates the use of my invention for the measurement of compression strains, it will be understood that I do not restrict myself to this particular application, but that the magnitude of other mechanical forces may be determined by the use of my invention. Various m0difications may therefore be made without departing from the nature of my invention as set forth in the appended claims.

I claim:

1. A method of measuring mechanical forces a series which consists in subjecting a body of magnetizable material to the forces under test. magnetizing said body by passing through an exciting winding,'an alternating current of such strength that the magnetization will exceed the limit given by the coercive force of the material of the body subjected to the forces under test, and measuring the impedance of said exciting winding.

2. A method of measuring mechanical forces which consists in subjecting a. body of magnetizable material to the forces under test, magnetizing said body by passing through an exciting winding, an alternating current of such strength that the product of the intensity of the current in said exciting winding multiplied by the variation in the reactance of said exciting winding in the strained body subjected 3. A method of measuring mechanical forces which consists in subjecting a body of magnetizable material to the forces under test, magnetizpedance of said exciting winding.

winning JANOVBKY. 

